The long-term goal of our project is to determine the role of Hox genes in the developing nervous system at the single-cell level. Ten leech Hox genes (called Lox genes) have been cloned and partially characterized. The Lox genes are homologous to genes of the vertebrate Hox complexes and of the insect homeotie complexes. These genes code for transcription factors that have a main site of expression in the central nervous system. Preliminary results showed that Lox genes are expressed in repeated sets of central neurons present in specific anterior-posterior domains. Within theft domains, they are expressed also in neurons that are not present in every ganglion, suggesting a role for Lox genes in the differentiation of these neurons. Central neurons and their target organs often express the same Lox genes. This observation suggests that Lox genes may play an important role in axonal guidance or target recognition. Our project seeks to address the following hypotheses: 1. Lox genes play a role in the determination of specific properties of central neurons. 2. Lox gene expression in central neurons is necessary for the normal innervation of their target organs. 3. Lox genes direct the expression of genes that control cell adhesion, axon guidance, and target recognition.